1. Field of the Invention
This invention relates in general to storage systems for petroleum products. Specifically, the invention relates to a subsea storage system and still more particularly, to a storage system which uses a membrane or bladder as part of its storage capacity for self-contained displacement water.
2. Description of the Prior Art
The continued growth of offshore oil and gas production far from shore has presented the problem of storing petroleum crude or processed petroleum at the production site. Such storing enables the producer to accumulate his output until a full tank or barge load is collected rather than to have to pump it directly into a barge or seagoing storage vessel on a daily basis. Such daily pumping would unnecessarily detain the vessel, because one day's production is generally less than a full load. Subsea storage is indicated because of the excessive cost of building offshore platforms far from shore to store crude oil above the sea surface, and because of the possible severe weather conditions encountered at sea.
In addition to the usual problems with which the oil producer is faced on land, environmental considerations are of great importance in connection with offshore drilling and production. Consequently, it is of paramount concern that adequate safety precautions be taken to prevent any leakage or spillage of petroleum products into the sea. As a result, production facilities located offshore are generally equipped with elaborate systems to insure that any seawater which becomes mixed with petroleum crude is filtered and cleaned prior to being returned to the sea.
One prior art attempt at solving the subsea petroleum storage problem is disclosed in U.S. Pat. No. 3,113,699 issued Dec. 10, 1963 to Crawford et al. Crawford discloses an underwater liquid storage system having one or more essentially fluid impermeable, flexible collapsible storage containers. Based upon the principle of water displacement, the containers change shape whenever liquids are introduced or withdrawn. The containers are horizontally oriented and disposed within a substantially rectangular rigid frame adapted to rest on the sea floor. The collapsible containers and enclosing frame are anchored to the sea floor to restrain them from movement due to buoyant forces and wave and current forces. The petroleum products are pumped directy into the flexible storage containers and then are discharged by the action of the sea pressure against the flexible container.
Another approach to the underwater storage system problem for petroleum products is described in U.S. Pat. No. 4,141,377 issued on Feb. 27, 1979 to Fernandez et al. Fernandez discloses an underwater storage assembly having at least one storage tank where the tank is maintained in position on a skid assembly so that one end of the tank is positioned higher than the other end. The tank includes at least one rigid cylindrical wall defining an inner chamber. A flexible membrane is secured to the inside of the inner cylindrical wall for dividing the inner chamber into first and second copartments to provide a fluid-tight compartment into a which a fluid can be received for storage and from which it can be discharged at a latter time. The tank is provided with a passage for allowing sea water to pass freely between the first compartment and the surrounding sea. A filtering apparatus associated with each tank is provided for preventing debris and sea organisms from entering a tank through its respective passage. The tank is provided at its higher end with piping for discharging fluids into and out of the membrane.
The prior art solutions to the underwater storage system problem for storing petroleum products have certain disadvantages. For example, with the Crawford apparatus should the flexible membrane develop a leak, petroleum products would immediately flow into the sea causing environmental damage. With the Fernandez invention, the use of sea water directly from the sea within the container requires expensive filtering apparatus to prevent marine organisms and silt from entering the container.